Such variable stroke engine has already been known as disclosed in Japanese Patent Application Laid-open No. 2005-54685 and the like.
However, in the variable stroke engine disclosed in Japanese Patent Application Laid-open No. 2005-54685, a driving gear for transmitting a power to a camshaft side and a driving gear for transmitting a power to a rotational shaft side are mounted on a crankshaft in a manner that these driving gears are adjacent to each other in the axial direction. Accordingly, the bearing span of the crankshaft is increased. This structure poses an increase in the diameter of the crankshaft for the purpose of avoiding deformation and an increase in friction of the crankshaft due to deterioration of the bearing support rigidity.